Sound signals arriving frontally at the ear are accentuated due to the shape of the pinna, which is the external portion of the ear. This effect is called directionality, and for the listener it improves the signal-to-noise ratio for sound signals arriving from the front direction compared to sound signals arriving from behind. Furthermore, the reflections from the pinna enhance the listener's ability to localize sounds. Sound localization may enhance speech intelligibility, which is important for distinguishing different sound signals such as speech signals, when sound signals from more than one direction in space are present. Localization cues used by the brain to localize sounds can be related to frequency dependent time and level differences of the sound signals entering the ear as well as reflections due to the shape of the pinna. E.g. at low frequencies, localization of sound is primarily determined by means of the interaural time difference.
For hearing aid users good sound localization and speech intelligibility may often be harder to obtain.
In some hearing aids, e.g. behind-the-ear (BTE) hearing aids, the hearing aid microphone is placed behind the external portion of the ear and therefore sound signals coming from behind and from the sides are not attenuated by the pinna. This is an unnatural sensation for the hearing aid user, because the shape of the pinna would normally only accentuate sound signals coming frontally.
Thus, a hearing aid user's ability to localize sound decreases as the hearing aid microphone is placed further away from the ear canal and thereby the eardrum. Thus sound localization may be degraded in BTE hearing aids compared to hearing aids such as in-the-ear (ITE) or completely-in-the-canal (CIC) hearing aids, where the microphone is placed closer to or in the ear canal.
In order to obtain an improved directionality, a directional microphone can be incorporated in hearing aids, e.g. in BTE hearing aids. The directional microphone can be more sensitive towards the sound signals arriving frontally in the ear of the hearing aid user and may therefore reproduce the natural function of the external portion of the ear, and a directional microphone therefore allows the hearing aid user to focus hearing primarily in the direction the user's head is facing. The directional microphone allows the hearing aid user to focus on whoever is directly in front of him/her and at the same time reducing the interference from sound signals, such as conversations, coming from the sides and from behind. A directional microphone can therefore be very useful in crowded places, where there are many sound signals coming from many directions, and when the hearing aid user wishes only to hear one person talking.
A directionality pattern or beamforming pattern may be obtained from at least two omni-directional microphones or at least one directional microphone in order to perform signal processing of the incoming sound signals in the hearing aid.
EP1414268 relates to the use of an ITE microphone to estimate a transfer function between ITE microphone and other microphones in order to correct the misplacement of the other microphones and in order to estimate the arrival direction of impinging signals.
US2005/0058312 relates to different ways to combine tree or more microphones in order to obtain directionality and reduce microphone noise. US2005/0041824 relates to level dependent choice of directionality pattern. A second order directionality pattern provides better directionality than a first order directionality pattern, but a disadvantage is more microphone noise. However, at high sound levels, this noise will be masked by the sound entering the hearing aid from the sides, and thus a choice between first and second order directionality can be made based on the sound level.
EP1005783 relates to estimating a direction-based time-frequency gain by comparing different beamformer patterns. The time delay between two microphones can be used to determine a frequency weighting (filtering) of an audio signal. EP1005783 describes using the comparison between a directional signal obtained from at least 2 microphone signals with the amplitude of one of the microphone signals.
“Binaural segregation in multisource reverberant environments” by N. Roman et al. describes a method of estimation a time-frequency mask by using a binaural segregation system that extracts the reverberant target signal from multisource reverberant mixtures by utilising only the location information of the target source.
“Enhanced microphone-array beamforming based on frequency-domain spatial analysis-synthesis” by M. M. Goodwin describes a delay-and-sum beamforming system in relation to distant-talking hands-free communication, where reverberation and interference from unwanted sound sources is hindering. The system improves the spatial selectivity by forming multiple steered beams and carrying out a spatial analysis of the acoustic scene. The analysis derives a time-frequency gain that, when applied to a reference look-direction beam, enhances target sources and improves rejection of interferers that are outside of the specified target region.
However, even though different prior art documents describe methods of how to improve sound localization in hearing aids, alternative methods of generating an audible signal in a hearing aid which may improve sound localization and speech intelligibility for the hearing aid user may be provided.